Summer Islands are a group of islands south of Betancuria. To get there from Betancuria, one has to travel down the Great River for about 5 days and a few more on the open sea.
The islands have a surprisingly warm climate, likely helped by sea currents. It can still be windy and rainy, but the warmth makes life much more bearable than the cold and icy weather in Betancuria. Most of the islands are quite small but all feature golden sandy beaches, lush semi-tropical forests, and knee-high grass. The only inhabitants are the wildlife, primarily made up of seabirds, mosquitoes, snakes, and even bears.
The Princess arrives on the largest of the Summer Islands during The Golden Chalice quest in order to explore the ruined Temple of Hyath, which can be found on the highest point of the Island.